1. Field of the Invention
The invention relates to aqueous rubber dispersions containing reinforcing multiphase polymer particles, which are used for the preparation of vulcanized latex foams having improved properties. In particular, the invention relates to dispersions of a reinforcing material which are prepared by a multistage emulsion polymerization process and have a predominant content or aromatic monovinyl compounds, the glass transition temperatures of the individual phases of the polymer particles being outside the temperature range in which the latex foams prepared therefrom are used.
2. Description of the Prior Art
Latex foam is prepared according to the prior art from aqueous, highly concentrated rubber dispersions with the addition of sulphur-crosslinking vulcanizing agents by mechanically beating in air, gelled by the addition of gelling agents and then vulcanized at elevated temperature to give a cellular substantially open-pore foam. The preparation of such latex foams is described, for example, in German Offenlegungsschrift 2,150,872. The rubber dispersions contain, as a rule, dispersions of a reinforcing material, i.e. aqueous dispersions of reinforcing polymers. The type and amount of this reinforcing component greatly influence the properties of the finished latex foam.
The latex foam is required to have high elasticity, in particular in the application temperature range of 20 to 70.degree. C., which elasticity must not be lost even during prolonged compression and especially in continuous use. Accordingly, the compression set (according to DIN 53 572) in this temperature range should be as small as possible and the recovery of the compressed shaped article should be as rapid as possible, i.e. the hysteresis (according to DIN 53 577) should likewise be as small as possible. Furthermore, the tensile strength and the maximum elongation at break (according to DIN 53 571) should be as high as possible. Likewise, at a predetermined indentation hardness (according to DIN 53 576) of the foam, its density should be as low as possible in order to be able to produce the foam articles as economically as possible with regard to the amount of rubber dispersions which has to be used.
Reinforcing polymers in the form of their aqueous dispersion have long been known for increasing the hardness, strength and elongation of latex foam in a controlled manner. These are as a rule styrene/butadiene dispersions having a high styrene content of up to 100%. Homogeneous dispersions of a reinforcing material, i.e., said dispersions prepared in a single process step, have already been described in German Offenlegungsschrift 1,056,364 as being disadvantageous since they do not meet the requirement for low values for the compression set over a wide temperature range. According to this teaching, better results are obtained by the use of multistage graft copolymer dispersions which have a comparable overall composition and are prepared in a two-stage batch process. Such rubber dispersions and latex foams prepared therefrom are distinguished by the fact that the glass transition temperature to be assigned to the reinforcing component is within a relatively narrow temperature range in the stated interval from 20 to 70.degree. C.
An embodiment of such a multiphase graft copolymer dispersion is described in European Patent 0,187,905, said embodiment being improved in important respects. There, use is made in particular of semicontinuous feed processes in order to prepare in a controlled manner dispersion particles having different monomer compositions in the individual phases. The latex foams prepared on the basis of this teaching are distinguished by a balanced combination of properties, with in particular low values for the compression set in the total temperature range from 20 to 70.degree. C. According to this embodiment, and in contrast to earlier teachings, the glass transition temperatures to be assigned to the individual phases of the reinforcing component cover as broad a range as possible within the stated interval from 20 to 70.degree. C. However, the poor resilience of the latex foams prepared according to this teaching is not completely satisfactory and is evident from the relatively high hysteresis values.
It is therefore an object of the present invention to provide dispersions of a reinforcing material for expandable rubber dispersions, with the aid of which dispersions it is possible to prepare latex foams which have a combination of properties which is improved over and above the prior art, and in particular have lower hysteresis values.